Maths in a minute: Gravitational waves

On February 11 2016 scientists of LIGO announced the first-ever
detection of gravitational waves. This was one of the most important
events in physics over the last few decades. But why? What are
gravitational waves and why are they so important?

Gravity is the manifestation of
the curvature of space and time. Image courtesy NASA.

As their name suggests, gravitational waves are connected to the force of gravity we are all familiar with.
In 1687 Isaac Newton
formulated his universal law of gravitation, which describes the gravitational attraction between two massive objects. The law remained unchallenged until 1905, when Albert Einstein published his special theory of relativity. The theory says that there is a universal speed limit in the Universe: nothing can travel faster than light, that is, nothing can travel faster than roughly 300,000 km per second. This contradicted Newton, who thought the effect of gravity was instantaneous: take away the Sun, and the effect will be felt on Earth immediately.

Einstein himself later remedied this problem by proposing that gravity isn't a force that wafts across the ether, but a result of the curvature of space. An analogy that is often given is that of a bowling ball sitting on a trampoline. The ball creates a dip in the trampoline, curving its surface, so a marble placed nearby will roll into the dip towards the ball. According to Einstein, massive bodies warp space in a similar way, causing less massive bodies to be attracted to them. (Find out more here.)

One of the consequences of Einstein's theory of gravity is that
when gravitational monsters such as black holes shunt their weight
around, they should create ripples that can be felt across space and
time. "Near black holes the curvature of space-time is extremely
high," explained gravity expert Bangalore Sathyaprakash (in
How does gravity
work?). "Now imagine two black holes moving around each
other: the curvature is large but also changing. It's a bit like
taking a stick and moving it around in a pond. That's going to
generate ripples in the water. Only in the case of black holes, we're
talking about ripples in the very fabric of spacetime."

A computer simulation of two black holes merging. The simulation shows the gravitational waves caused by this event. Credit: Max-Planck-Institut für Gravitationsphysik (Albert-Einstein-Institut).

That these ripples should exist is important for theoretical reasons: not only is Einstein's theory correct in predicting those waves, tests performed on the gravitational waves that have been
detected
have also confirmed that all
gravitational waves travel at the speed of light. This rules out
alternative theories of gravity in which this isn't the case.

But gravitational wave are also important for another reason: they
give us a new way of listening to the Universe, and finding out about
the objects that lurk within it. "The most exciting thing is what
happens next," said David Tong, a theoretical physicist at the
University of Cambridge. "In the near future, we could be seeing dozens of these events each year: black holes colliding, neutron stars colliding, and who knows what else. We have a new way of looking at the Universe. It's going to teach us many things."

The following video shows Gabriela González, Professor of Physics and Astronomy at Louisiana State University and former Spokesperson of the LIGO Scientific Collaboration, explaining gravitational waves.